More than any other animal except humans, beavers geomorphically alter the landscape through their dam building and related activities. This chapter examines the geomorphic effects of beavers with special emphasis on the North American landscape, where beaver were historically widespread and numerous, and where they have recently reoccupied their historic range.

Beaver species and morphology

Two closely related species of beaver comprise the modern genus Castor: Castor fiber, the native European beaver, and Castor canadensis, the North American beaver (Williams 1988). Although a large body of literature exists on the ethology and morphology of Castor fiber, much more attention has been given to the geomorphic effects of beavers in North America. Studies of dam building and the geomorphic influences of beavers in Europe (cf. Wilsson 1971; Sinitsyn and Rusanov 1990; A. Meadows 1991; Zurowski 1992) also suggest that little if any difference exists in the constructional activities of the two species and the corresponding geomorphic results; accordingly, except where specifically noted, the following discussion focuses specifically on Castor canadensis and its geomorphic influence on the landscape.

The North American beaver (C. canadensis) is a large (adult mass > 15 kg), herbivorous, semiaquatic rodent with webbed rear feet, a flat and scalyappearing tail, and long front incisors used as chisels for chewing through wood (Avery 1983; Novak 1987; Scheid 1987; Ryden 1989; Butler 1991a; Reynolds 1993).